Spray foam over breathable membrane: a problem?

Under the tiles or slates of most modern roofs sits an underlay. Older bitumen felts are largely vapour-closed, which is why a cold roof needs a ventilated gap above the insulation. Newer breathable membranes, also called low-resistance or LR underlays, let water vapour pass through them, so a roof can dry upward and outward.

Marley's guidance on cold-roof ventilation, which cites BS 5250, shows the difference. Where the underlay is low-resistance and the ceiling is well sealed, it says fortuitous ventilation at the eaves, about 3 mm of normal construction, is all that is required, or a 5 mm opening at the ridge. A vapour-closed felt needs far more, with 25 mm at the eaves on low pitches.

Those figures assume a roof that can still pass vapour. The membrane is only half of a system, and the other half is a path for the vapour to travel to it.

Sprayed to the underside of the membrane or battens, foam sits where the vapour would have travelled. Closed-cell foam resists vapour strongly, so the membrane has little to dry. Open-cell foam is more permeable, yet it can still hold water and keep timber damp for long periods. Our guide to spray foam, condensation and timber decay sets out how that damp builds up.

Where foam has been applied over the membrane and rafters together, the rafters are encased. Moisture that enters from the room or leaks through the covering has few places to leave. The RICS consumer guide to spray foam says that poor installation can cause dampness, such as uncontrolled condensation, and recommends condensation risk checks by a qualified professional.

Each build-up is slightly different, and roofs differ in what was done to them after the foam went on, which is why a general statement that foam over membrane is always bad would be too blunt. The installation, the foam type and the condition of the timber all count.

Some signs show from the loft or the room below. Staining on plasterboard, a musty smell, dark marks where rafters meet the foam and condensation on nail heads or metal fixings all point to moisture in the wrong place. From outside, slipped or lifting battens can follow if the timber is softening.

Treat none of these as proof of decay, and an absence of signs proves nothing either, because the foam covers the timber. That is why we use readings. A moisture meter on the exposed timber at an opened section gives a number, and a few readings along a rafter show whether wetness is local or spread.

Our page on how messy spray foam removal is explains what the exposure of a section involves, so you can decide whether an opening is acceptable before a survey begins.

What to photograph yourself

Take pictures of any staining, the loft hatch area and the eaves, with a date. They help whoever inspects the roof compare the roof now with how it looked before.

If readings are low and the foam is not in contact with damp timber, monitoring may be reasonable. Ventilation is rarely a clean answer here, for reasons given in our piece on adding ventilation instead of removing foam. Where readings are high, removal from the affected bays is the usual response, followed by new insulation that lets vapour move, such as mineral wool or wood fibre laid with an air path above where the design needs one.

Re-insulation must meet current rules. Where a roof is renovated enough to trigger Building Regulations, Part L expects the element to be improved, and we explain it before work begins.

Timber that is sound is left; timber that has softened may need cutting out and sistering, meaning a new length fixed alongside it. We would show you any such item in photographs and in the quote, so that a repair to a rafter is never folded in without you knowing.

Where a roof is stripped to the membrane, there is a chance to check the membrane itself too. Tears, tight laps and nail holes around battens are all easier to deal with while the covering is off, and a damaged membrane over a roof that has just been cleared of foam would undo some of the benefit of the work.

Ask who installed the foam, whether the membrane was left in place and whether it was cut or lifted at any point during the job. Ask whether the roof had a covering renewed afterwards, and whether any reports or readings exist. A buyer should put the same questions to the seller through the solicitor. Our guide for buying a house in Stroud with spray foam lists them.

A specialist assessment typically costs £100 to £200 according to MyBuilder's 2026 guide, which is small next to removal at about £20 to £50 per m². Send a few photographs of what you have and we will explain what a survey would check on this particular build-up. If work is needed, the written quote that follows sets out what is involved, and anything we do carries our 10-year workmanship guarantee in writing. See spray foam insulation removal for the whole service.